Question

Define valence electrons. Why do helium ,neon,argon have zero valency

Answer 1

In chemistry, a valence electron is an outer shell electron that is associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed; in a single covalent bond, both atoms in the bond contribute one valence electron in order to form a shared pair.




 
Octet means that outer most orbit hascomplete eight electrons to stay stable. Atoms lose or gain electrons to become stable. The noble gases are already stable, so their tendency to gain or lose electrons is zero. Hence, their valency is also zero.

Answer 2

The combining capacity of an element is called as valency.
Helium neon argon have zero valency because the are stable atom thus they do not
take part in reaction.
Therefore its valency is zero